In LTE (Long Term Evolution), which is one of the standards for radio communication systems defined by 3GPP (3rd Generation Partnership Project), radio resources including a time domain and a frequency domain are assigned to each radio terminal (User Equipment: UE) by using the TDM (Time Domain Multiplexing)/FDM (Frequency Domain Multiplexing) scheme. With respect to uplink signals transmitted by multiple radio terminals to a radio base station (enhanced Node B: eNB) in particular, the radio base station controls the transmission timing of an uplink signal of each radio terminal so that it is accommodated within a predetermined receive window at the radio base station. This control of uplink-signal transmission timing is performed by using the following two (NPL 1).                Uplink-signal transmission timing adjustment value (Timing Advance: TA)        Uplink-signal synchronization timer (Time Alignment Timer: TAT)        
The transmission timing adjustment value TA is information indicating a value for a radio terminal to advance or delay the current transmission timing by a predetermined amount. The synchronization timer TAT indicates a duration for which the timing of receiving an uplink signal at a radio base station is accommodated within a predetermined window, that is, uplink-signal synchronization is guaranteed, with a transmission timing currently configured. The radio terminal is enabled to transmit uplink signals while the synchronization timer TAT is running, but does not transmit (is disabled to transmit) uplink signals when the synchronization timer TAT expires.
Moreover, in LTE-Advanced (LTE-A), which is a radio communications system advanced from LTE, the standardization of carrier aggregation (CA) is being proceeded, in which radio terminals use multiple component carriers (CC) at the same time to transmit and receive user data and the like (NPL 2). Each component carrier CC corresponds to one system bandwidth defined in LTE and can be thought to correspond to one cell. That is, a downlink component carrier CC and a corresponding uplink component carrier CC in combination are thought to be one cell. For example, transmission and reception on two downlink (or uplink) component carriers CC can be translated to transmission and reception on two cells. Accordingly, communication using a single uplink/downlink component carrier CC corresponds to communication on a single cell, and in the description hereinafter, both or one of the component carrier and the cell will be used appropriately.
Here, a component carrier CC that performs the most basic functions, such as obtaining system information required for a radio terminal to communicate with a radio base station, is referred to as primary component carrier (Primary CC: PCC) or primary cell (PCell), and other component carriers are referred to as secondary component carrier (Secondary CC: SCC) or secondary cell (SCell).
In LTE-A, studies have hitherto been proceeded on the premise that when carrier aggregation CA is performed, common uplink-signal transmission timing is used on multiple component carriers CC or multiple cells. That is, even when uplink signals are transmitted by using multiple component carriers (multiple cells), there is one uplink transmission timing adjustment value TA that a radio base station notifies to a radio terminal at certain time, and there also is one synchronization timer TAT for each radio terminal. Thereby, it is possible to easily perform uplink-signal transmission timing control without complicating the control even when carrier aggregation CA is performed.
On the other hand, in 3GPP, studies of a technology improved from the technology standardized as LTE-A have been started. Specifically, the technology has been discussed that makes carrier aggregation CA feasible even if uplink-signal transmission timing differs between a plurality of component carriers CC, that is, a plurality of cells. Factors causing uplink-signal transmission timing to differ between a plurality of component carriers CC (a plurality of cells) are different frequency bands, a repeater (reproduction station) being set for each frequency band (or only for a specific frequency band), and the like.
In 3GPP, a group of one or a plurality of component carriers (cells) on which uplink-signal transmission timing can be controlled in common is referred to as synchronization group (Timing Advance Group: TA Group). NPL 3 proposes a method for uplink-signal transmission timing control, in which the uplink-signal transmission timing adjustment value TA is controlled for each such TA Group on which timing control can be performed in common, and in which one common synchronization timer TAT is maintained for each radio terminal.
Hereinafter, a brief description will be given of a method for uplink-signal transmission timing control in a case where uplink-signal transmission timing differs between a plurality of component carriers CC (a plurality of cells), with reference to FIGS. 1 to 3.
Referring to FIG. 1, a system will be considered in which two TA Groups 1 and 2 with different uplink-signal transmission timings exist. Here, it is assumed that a primary cell PCell and a secondary cell SCell1 belong to the same TA Group 1, and secondary cells SCell2 and SCell3 belong to the same TA Group 2 as shown in FIG. 2A, and that to a radio base station eNB, a radio terminal UE performs uplink transmission by using the three secondary cells SCell1-3 in addition to the primary cell PCell. In this case, since uplink transmission timing differs between the TA Groups 1 and 2, uplink transmission timing adjustment values TA1 and TA2 for the respective TA Groups are configured so that uplink-signal reception timing at the radio base station eNB will be accommodated within a predetermined window as shown in FIG. 2B.
Moreover, referring to FIG. 3, according to the method for uplink transmission timing control (NPL 3), a radio terminal UE controls its synchronization timer TAT, linking it with uplink-signal transmission timing control on the TA Group 1 that includes the primary cell PCell. That is, the synchronization timer TAT is started upon establishment of uplink-signal synchronization in the TA Group 1. Thereafter, the synchronization timer TAT is started from an initial value each time an uplink-signal transmission timing adjustment value TA1 for the TA Group 1 is received (hereinafter, this will simply be referred to as “restart”).
On the other hand, uplink-signal transmission timing control on the TA Group 2 is performed independently of the TA Group 1. When the radio base station eNB determines that an uplink transmission timing adjustment value TA2 for the TA Group 2 needs to be updated, the radio base station eNB notifies a transmission timing adjustment value TA2 to the radio terminal UE, and the radio terminal UE applies the received transmission timing adjustment value TA2. According to this control, the radio terminal UE commits uplink-signal synchronization in the TA Group 2 to the radio base station eNB. Accordingly, the radio base station eNB needs to appropriately update a transmission timing adjustment value TA2 so that uplink-signal synchronization in the TA Group 2 is guaranteed (details are all dependent on the implementation of the radio base station eNB). Thus, it is possible to transmit uplink signals at the same time in TA Groups with different uplink transmission timings, without increasing the load on a radio terminal UE.